The present invention relates to a heating method and apparatus for carrying out residual-stress-relief treatment by induction heating (Induction heating stress improving treatment) of a pipe line system in industrial plants and nuclear power plants under construction or in operation especially of welded joints between main and branch pipes and portions adjacent to the welded joints.
Recently, such residual-stress-relief treatment has been widely carried out in a pipe line of a nuclear power plant under construction or in operation in order to eliminate residual tensile stress caused in inner surfaces of pipes by thermal stress on the pipes joined together by welding or to change such residual stress into compression stress.
When the welded joint between pipes in a pipe line is left as-welded, carbide is precipitated in grain boundaries of the surfaces adjacent to the welded joint so that the microstructure sensitizes and the residual stress is caused in the inner surfaces of the pipes adjacent to the welded joint therebetween. When the operation of a plant is started under these conditions and, for instance, a high-temperature and high-pressure liquid is forced to flow through the pipe line, corrosive components in the liquid synergetically effect with the sensitized structure and the residual stress so that intergranular corrosion cracks result at the portions adjacent to the welded joint. The residual-stress-relief treatment is made so as to overcome such intergranular corrosion cracks as described above.
The residual-stress-relief treatment, which is directed to elimination of the residual tensile stress caused in the inner surfaces of the pipes or change of the same into the compression stress, is carried out as follows. First, while a liquid flow is forced through a pipe line so as to cool inner surfaces of pipes, a suitable external heating device is used to locally heat only the welded joint and a portion adjacent thereto so that a predetermined temperature difference results between the outer and inner surfaces of the pipes. As a result, the thermal stress in excess of the yield point is caused at the heated portion. Thereafter, the heated portion is cooled to room temperature so that the temperature difference between the outer and inner surfaces of the pipes can be eliminated. When such residual-stress-relief treatment is carried out in practice in a plant, there arise the following problems.
For instance, when a welded joint is subjected to the residual-stress-relief treatment, the temperature of all the heated portion must not exceed a critical temperature and the temperature difference between the outer and inner surfaces of a predetermined portion including the welded joint (for instance, a portion 3.sqroot.Rt in length in the axial direction of the joined pipes, where R is the radius of the pipes and t is the thickness of the pipes) must be raised to a level at which the residual stress can be relieved. To this end, the shapes of the welded joint and a predetermined portion adjacent thereto must be investigated in full detail and then configuration of an inductor which can attain a most preferable temperature distribution must be determined before a best suitable induction for carrying out the residual-stress-relief treatment of this specific welded joint is fabricated.
However, component parts which constitute a pipe line and which must be joined to each other by welding include not only straight pipes but also elbows, tees, crosses, valves, pumps, sweepolets (trade name: set-in type nozzle), weldolets (trade name: set-out type nozzle), caps and so on. The actual shapes and precise sizes of these component parts cannot be obtained from drawings thereof (because the sizes of these component parts described on their drawings are the so-called design sizes or reference sizes which determine the maximum or minimum allowable sizes so that the design sizes might not coincide with the actual sizes). As a result, in the case of the fabrication of an inductor, the shapes, sizes and other required data of these components must be measured in the field and an inductor suitable for a specific component part must be fabricated based upon the actually measured data as described above. As a result, much time and labor are needed.
Especially in the case of a nuclear power plant in operation, the shapes and sizes of these component parts must be measured only during a shutdown period (a few months) in which nuclear fuel elements are replaced and which is cycled every one or one and a half years generally. Furthermore, it takes about four months to obtain the above-described data, design and fabricate an inductor so that it is impossible to carry out the residual-stress-relief treatment during one shutdown period. As a result, the residual-stress-relief treatment is carried out in practice during the next shutdown period.
If there is a danger that intergranular corrosion cracks would be caused until the next shutdown period, the previous or first shutdown period must be increased by about two months so that the complete residual-stress-relief treatment can be carried out. Such increase in shutdown period usually results in a fuel difference loss of as high as billions yen (that is, a difference in cost between nuclear fuels and heavy oil or coal fuels), so that a tremendous economical loss is incurred.
Furthermore, in the case of the residual-stress-relief treatment of a pipe seat (a member interposed between a main pipe and a branch pipe when they are joined together), not only the pipe seat but also a predetermined portion of the main pipe adjacent to the welded joint between the main pipe and the pipe seat and a predetermined portion of the branch pipe adjacent to the welded joint between the branch pipe and the pipe seat as well must be heated to a predetermined temperature. In this case, even when the actual shapes and sizes of the main pipe, the pipe seat and the branch pipe are available, the assembly of the main pipe, the pipe seat and the branch pipe is very complicated in shape. As a consequence, it is impossible in practice to uniformly heat such assembly. Therefore, in practice, a full-size mock-up of the assembly must be fabricated based upon the actually measured data, and the shape and size of an inductor are modified based upon this full size mock-up so that a suitable inductor capable of uniformly heating the assembly must be designed and fabricated. That is, the so-called mock-up tests must be carried out to design and fabricate an optimum inductor for carrying out the residual-stress-relief treatment of the assembly.
However, the mock-up tests need much labor and much time so that it is impossible in practice during one shutdown period to measure the shapes and sizes of component parts which will be subjected to the residual-stress-relief treatment, then to carry out the mock-up tests in the manner described above and to design and fabricate an optimum inductor.
Furthermore, in the case of a conventional inductor used to carry out the residual-stress-relief treatment which is designed and fabricated after the mock-up tests based upon the actually measured data of component parts which will be subjected to the residual-stress-relief treatment, the pitches between the coils of the inductor and the clearances between the coils and the surfaces of the component parts are fixed so that it is impossible to change them even when a desired surface temperature distribution cannot be obtained in the residual-stress-relief treatment.
Meanwhile, it has been well known in the art that when a portion of the outer surface of a pipe is heated substantially uniformly by induction heating or the like while water is forced to flow through the pipe, the temperature of this portion is in proportion to the thickness of the wall of the pipe. Therefore in the case of the conventional inductor used in the residual-stress-relief treatment, a clearance between a predetermined portion of a pipe and an induction coil is increased and/or the pitch of the induction coils is increased when it is expected that the temperature of said predetermined portion rises in excess of a predetermined temperature because the thickness of the wall of this portion is thick. However, as described hereinbefore, even when the clearances and pitches are found to be not satisfactory for attaining a desired temperature distribution, it is impossible to change them.
In view of the above, the present invention has for its object to provide a heating method and apparatus capable of uniformly heating a welded joint and portions adjacent thereto in a pipe line in a nuclear power plant under construction or in operation without the need of measuring the actual shapes and sizes of component parts and of designing and constructing an inductor through the mock-up tests in the manner described above.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawings.